Note:
This paragraph is informative. This document is currently not
maintained. The CSS Working Group is developing CSS Level 2
Revision 1, which has much more precise and Web-compatible
definitions of the features described here. The CSS Working Group
encourages authors and implementors to reference CSS 2.1 (or its
successor) instead
of this document and when features common to CSS1 and CSS 2.1 are
defined differently to follow the definitions in
CSS 2.1.

Status of this document

This document is a W3C Recommendation. It has been reviewed by W3C (http://www.w3.org/) Members and
general consensus that the specification is appropriate for use has
been reached. It is a stable document and may be used as reference
material or cited as a normative reference from another document. W3C
promotes widespread deployment of this Recommendation.

This document specifies level 1 of the Cascading Style Sheet
mechanism (CSS1). CSS1 is a simple style sheet mechanism that allows
authors and readers to attach style (e.g. fonts, colors and spacing)
to HTML documents. The CSS1 language is human readable and writable,
and expresses style in common desktop publishing terminology.

One of the fundamental features of CSS is that style sheets
cascade; authors can attach a preferred style sheet, while the reader
may have a personal style sheet to adjust for human or technological
handicaps. The rules for resolving conflicts between different style
sheets are defined in this specification.

This Recommendation results from W3C activities in the area of Style Sheets. For
background information on style sheets, see [1].

a selector that matches elements based on their position in the
document structure. A contextual selector consists of several
simple selectors. E.g., the contextual selector 'H1.initial B'
consists of two simple selectors, 'H1.initial' and 'B'.

CSS

Cascading Style Sheets

CSS1

Cascading Style Sheets, level 1. This document defines CSS1
which is a simple style sheet mechanism for the web.

CSS1 advanced features

features that are described in this specification but labeled as
not among the CSS1 core features

a tool for describing the behavior of pseudo-classes and
pseudo-elements

font size

The size for which a font is designed. Typically, the size of a
font is approximately equal to the distance from the bottom of
the lowest letter with a descender to the top of the tallest
letter with an ascender and (optionally) with a diacritical
mark.

Markup introduced by UA vendors, most often to support certain
visual effects. The "FONT", "CENTER" and "BLINK" elements are
examples of HTML extensions, as is the "BGCOLOR" attribute. One
of the goals of CSS is to provide an alternative to HTML
extensions.

inline element

an element which does not have a line break before and after
(e.g. 'STRONG' in HTML)

intrinsic dimensions

the width and height as defined by the element itself, not
imposed by the surroundings. In this specification it is assumed
that all replaced elements -- and only replaced elements -- come
with intrinsic dimensions.

pseudo-elements are used in CSS selectors to address
typographical items (e.g. the first line of an element) rather
than structural elements.

pseudo-class

pseudo-classes are used in CSS selectors to allow information
external to the HTML source (e.g. the fact that an anchor has
been visited or not) to classify elements.

property

a stylistic parameter that can be influenced through CSS. This
specification defines a list of properties and their
corresponding values.

reader

the person for whom the document is rendered

replaced element

an element that the CSS formatter only knows the intrinsic
dimensions of. In HTML, 'IMG', 'INPUT', 'TEXTAREA', 'SELECT' and
'OBJECT' elements can be examples of replaced elements. E.g.,
the content of the 'IMG' element is often replaced by the image
that the SRC attribute points to. CSS1 does not define how the
intrinsic dimensions are found.

rule

a declaration (e.g. 'font-family: helvetica') and its selector
(e.g. 'H1')

selector

a string that identifies what elements the corresponding rule
applies to. A selector can either be a simple selector (e.g.
'H1') or a contextual selector (e.g. 'H1 B') which consists of
several simple selectors.

SGML

Standard Generalized Markup Language [5], of
which HTML is an application

simple selector

a selector that matches elements based on the element type
and/or attributes, and not he element's position in the document
structure. E.g., 'H1.initial' is a simple selector.

style sheet

a collection of rules

UA

User Agent, often a web browser or web client

user

synonymous with reader

weight

the priority of a rule

In the text of this specification, single quotes ('...') denote
HTML and CSS excerpts.

Designing simple style sheets is easy. One needs only to know a
little HTML and some basic desktop publishing terminology. E.g., to
set the text color of 'H1' elements to blue, one can say:

H1 { color: blue }

The example above is a simple CSS rule. A rule consists of two main
parts: selector ('H1') and declaration ('color: blue'). The
declaration has two parts: property ('color') and value ('blue').
While the example above tries to influence only one of the properties
needed for rendering an HTML document, it qualifies as a style sheet
on its own. Combined with other style sheets (one fundamental feature
of CSS is that style sheets are combined) it will determine the final
presentation of the document.

The selector is the link between the HTML document and the style
sheet, and all HTML element types are possible selectors. HTML element
types are defined in the HTML specification [2].

HTML authors need to write style sheets only if they want to
suggest a specific style for their documents. Each User Agent (UA,
often a "web browser" or "web client") will have a default style sheet
that presents documents in a reasonable -- but arguably mundane --
manner. Appendix A contains a sample style
sheet to present HTML documents as suggested in the HTML 2.0
specification [3].

In order for the style sheets to influence the presentation, the UA
must be aware of their existence. The HTML specification [2] define how to link HTML with style sheets. This
section is therefore informative, but not normative:

The example shows four ways to combine style and HTML: using the
'LINK' element to link an external style sheet, a 'STYLE' element
inside the 'HEAD' element, an imported style sheet using the CSS
'@import' notation, and a 'STYLE' attribute on an element inside
'BODY'. The latter option mixes style with content and loses the
corresponding advantages of traditional style sheets.

The 'LINK' element references alternative style sheets that the
reader can select, while imported style sheets are automatically
merged with the rest of the style sheet.

Traditionally, UAs have silently ignored unknown tags. As a result, old UAs will
ignore the 'STYLE' element, but its content will be treated as part of
the document body, and rendered as such. During a transition phase,
'STYLE' element content may be hidden using SGML comments:

<STYLE TYPE="text/css"><!--
H1 { color: green }
--></STYLE>

Since the 'STYLE' element is declared as "CDATA" in the DTD (as
defined in [2]), conforming SGML parsers will not
consider the above style sheet to be a comment that is to be removed.

In the first example, the color of 'H1' elements was set to blue.
Suppose there is an 'H1' element with an emphasized element inside:

<H1>The headline <EM>is</EM> important!</H1>

If no color has been assigned to the 'EM' element, the emphasized
"is" will inherit the color of the parent element, i.e. it will also
appear in blue. Other style properties are likewise inherited, e.g. 'font-family' and 'font-size'.

To set a "default" style property for a document, one can set the
property on an element from which all visible elements descend. In
HTML documents, the 'BODY' element can serve this function:

BODY {
color: black;
background: url(texture.gif) white;
}

This will work even if the author has omitted the 'BODY' tag (which
is legal) since the HTML parser will infer the missing tag. The
example above sets the text color to be black and the background to be
an image. The background will be white if the image is not available.
(See section 5.3 for more on this.)

Some style properties are not inherited from the parent element to
the child element. Most often it is intuitive why this is not the
case. E.g., the 'background' property does
not inherit, but the parent element's background will shine through by
default.

Often, the value of a property is a percentage that refers to
another property:

To increase the granularity of control over elements, a new
attribute has been added to HTML [2]: 'CLASS'. All
elements inside the 'BODY' element can be classed, and the class can
be addressed in the style sheet:

The normal inheritance rules apply to classed elements; they
inherit values from their parent in the document structure.

One can address all elements of the same class by omitting the tag
name in the selector:

.pastoral { color: green } /* all elements with CLASS pastoral */

Only one class can be specified per selector. 'P.pastoral.marine'
is therefore an invalid selector in CSS1. (Contextual selectors,
described below, can have one class per simple selector)

CSS gives so much power to the CLASS attribute, that in many cases
it doesn't even matter what HTML element the class is set on -- you
can make any element emulate almost any other. Relying on this power
is not recommended, since it removes the level of structure that has a
universal meaning (HTML elements). A structure based on CLASS is only
useful within a restricted domain, where the meaning of a class has
been mutually agreed upon.

HTML [2] also introduces the 'ID' attribute
which is guaranteed to have a unique value over the document. It can
therefore be of special importance as a style sheet selector, and can
be addressed with a preceding '#':

In the above example, the first selector matches the 'P' element
due to the 'ID' attribute value. The second selector specifies both an
element type ('H1') and an ID value, and will therefore not match the
'P' element.

By using the ID attribute as selector, one can set style properties
on a per-element basis. While style sheets have been designed to
augment document structure, this feature will allow authors to create
documents that present well on the canvas without taking advantage of
the structural elements of HTML. This use of style sheets is
discouraged.

Inheritance saves CSS designers typing. Instead of setting all
style properties, one can create defaults and then list the
exceptions. To give 'EM' elements within 'H1' a different color, one
may specify:

H1 { color: blue }
EM { color: red }

When this style sheet is in effect, all emphasized sections within
or outside 'H1' will turn red. Probably, one wanted only 'EM' elements
within 'H1' to turn red and this can be specified with:

H1 EM { color: red }

The selector is now a search pattern on the stack of open elements,
and this type of selector is referred to as a contextual
selector. Contextual selectors consist of several simple
selectors separated by whitespace (all selectors described up to now
have been simple selectors). Only elements that match the last simple
selector (in this case the 'EM' element) are addressed, and only if
the search pattern matches. Contextual selectors in CSS1 look for
ancestor relationships, but other relationships (e.g. parent-child)
may be introduced in later revisions. In the example above, the search
pattern matches if 'EM' is a descendant of 'H1', i.e. if 'EM' is
inside an 'H1' element.

UL LI { font-size: small }
UL UL LI { font-size: x-small }

Here, the first selector matches 'LI' elements with at least one
'UL' ancestor. The second selector matches a subset of the first, i.e.
'LI' elements with at least two 'UL' ancestors. The conflict is
resolved by the second selector being more specific because of the
longer search pattern. See the cascading
order (section 3.2) for more on this.

The first selector matches all 'P' elements that have a 'DIV' among
the ancestors. The second selector
matches all 'H1' elements that have an ancestor of class
'reddish'. The third selector matches all 'CODE' elements that
are descendants of the element with 'ID=x78y'. The fourth selector
matches all 'H1' elements that have a 'DIV' ancestor with class
'sidenote'.

In CSS1, style is normally attached to an element based on its
position in the document structure. This simple model is sufficient
for a wide variety of styles, but doesn't cover some common effects.
The concept of pseudo-classes and pseudo-elements extend addressing in
CSS1 to allow external information to influence the formatting
process.

Pseudo-classes and pseudo-elements can be used in CSS selectors,
but do not exist in the HTML source. Rather, they are "inserted" by
the UA under certain conditions to be used for addressing in style
sheets. They are referred to as "classes" and "elements" since this is
a convenient way of describing their behavior. More specifically,
their behavior is defined by a fictional tag sequence.

Pseudo-elements are used to address sub-parts of elements, while
pseudo-classes allow style sheets to differentiate between different
element types.

All 'A' elements with an 'HREF' attribute will be put into one and
only one of these groups (i.e. target anchors are not affected). UAs
may choose to move an element from 'visited' to 'link' after a certain
time. An 'active' link is one that is currently being selected (e.g.
by a mouse button press) by the reader.

The formatting of an anchor pseudo-class is as if the class had
been inserted manually. A UA is not required to reformat a currently
displayed document due to anchor pseudo-class transitions. E.g., a
style sheet can legally specify that the 'font-size' of an 'active'
link should be larger than a 'visited' link, but the UA is not
required to dynamically reformat the document when the reader selects
the 'visited' link.

Pseudo-class selectors do not match normal classes, and vice versa.
The style rule in the example below will therefore not have any
influence:

A:link { color: red }
<A CLASS=link NAME=target5> ... </A>

In CSS1, anchor pseudo-classes have no effect on elements other
than 'A'. Therefore, the element type can be omitted from the
selector:

Some common typographical effects are associated not with
structural elements but rather with typographical items as formatted
on the canvas. In CSS1, two such typographical items can be addressed
through pseudo-elements: the first line of an element, and the first
letter.

CSS1 core: UAs may ignore all rules with ':first-line' or
':first-letter' in the selector, or, alternatively, only support a
subset of the properties on these pseudo-elements. (see section 7)

The 'first-letter' pseudo-element is used for "initial caps" and
"drop caps", which are common typographical effects. It is similar to
an inline element if its 'float' property is 'none', otherwise it is
similar to a floating element. These are the properties that apply to
'first-letter' pseudo-elements: font properties (5.2), color and background properties (5.3), 'text-decoration'
(5.4.3), 'vertical-align' (only if 'float' is 'none', 5.4.4), 'text-transform' (5.4.5), 'line-height' (5.4.8), margin properties (5.5.1-5.5.5), padding
properties (5.5.6-5.5.10), border properties (5.5.11-5.5.22),
'float' (5.5.25), 'clear' (5.5.26).

If a text-based UA supports the 'first-letter' pseudo-element (they
probably will not), the above could be formatted as:

___
| HE FIRST few
| words of an
article in the
Economist.

The fictional tag sequence is:

<P>
<SPAN>
<P:first-letter>
T
</P:first-letter>he first
</SPAN>
few words of an article in the Economist.
</P>

Note that the 'first-letter' pseudo-element tags abut the content
(i.e. the initial character), while the 'first-line' pseudo-element
start tag is inserted right after the start tag of the element it is
attached to.

The UA defines what characters are inside the 'first-letter'
element. Normally, quotes that precede the first letter should be
included:

|| /\ bird in
/ \ the hand
/----\ is worth
/ \ two in
the bush," says an
old proverb.

When the paragraph starts with other punctuation (e.g. parenthesis
and ellipsis points) or other characters that are normally not
considered letters (e.g. digits and mathematical symbols),
'first-letter' pseudo-elements are usually ignored.

Some languages may have specific rules about how to treat certain
letter combinations. In Dutch, for example, if the letter combination
"ij" appears at the beginning of a word, they should both be
considered within the 'first-letter' pseudo-element.

The 'first-letter' pseudo-element can only be attached to a
block-level element.

The above example would make the first letter of all 'P' elements
with 'CLASS=initial' red. When combined with classes or
pseudo-classes, pseudo-elements must be specified at the end of the
selector. Only one pseudo-element can be specified per selector.

In this example, the first letter of each 'P' element would be
green with a font size of 24pt. The rest of the first line (as
formatted on the canvas) would be blue while the rest of the paragraph
would be red. Assuming that a line break will occur before the word
"ends", the fictional tag sequence is:

Note that the 'first-letter' element is inside the 'first-line'
element. Properties set on 'first-line' will be inherited by
'first-letter', but are overridden if the same property is set on
'first-letter'.

If a pseudo-element breaks up a real element the necessary extra
tags must be regenerated in the fictional tag sequence. For example,
if a SPAN element spans over a </P:first-line> tag, a
set of SPAN end and start tags must be regenerated and the fictional
tag sequence becomes:

Both readers and authors can influence the presentation through
style sheets. To do so, they use the same style sheet language
thus reflecting a fundamental feature of the web: everyone can become
a publisher. The UA is free to choose the mechanism for
referencing personal style sheets.

Sometimes conflicts will arise between the style sheets that
influence the presentation. Conflict resolution is based on each style
rule having a weight. By default, the weights of the reader's rules
are less than the weights of rules in the author's documents. I.e., if
there are conflicts between the style sheets of an incoming document
and the reader's personal sheets, the author's rules will be used.
Both reader and author rules override the UA's default values.

The imported style sheets also cascade with each other, in the
order they are imported, according to the cascading rules defined
below. Any rules specified in the style sheet itself override rules in
imported style sheets. That is, imported style sheets are lower in the
cascading order than rules in the style sheet itself. Imported style
sheets can themselves import and override other style sheets,
recursively.

In CSS1, all '@import' statements must occur at the start of a
style sheet, before any declarations. This makes it easy to see that
rules in the style sheet itself override rules in the imported style
sheets.

In the example above, the first three declarations have increased
weight, while the last declaration has normal weight.

A reader rule with an important declaration will override an author
rule with a normal declaration. An author rule with an important
declaration will override a reader rule with an important declaration.

Conflicting rules are intrinsic to the CSS mechanism. To find the
value for an element/property combination, the following algorithm
must be followed:

Find all declarations that apply to the element/property in
question. Declarations apply if the selector matches the element in
question. If no declarations apply, the inherited value is used. If
there is no inherited value (this is the case for the 'HTML' element
and for properties that do not inherit), the initial value is used.

Sort by origin: the author's style sheets override the reader's
style sheet which override the UA's default values. An imported style
sheet has the same origin as the style sheet from which it is
imported.

Sort by specificity of selector: more specific selectors will
override more general ones. To find the specificity, count the number
of ID attributes in the selector (a), the number of CLASS attributes
in the selector (b), and the number of tag names in the selector (c).
Concatenating the three numbers (in a number system with a large base)
gives the specificity. Some examples:

Pseudo-elements and pseudo-classes are counted as normal elements
and classes, respectively.

Sort by order specified: if two rules have the same weight, the
latter specified wins. Rules in imported style sheets are considered
to be before any rules in the style sheet itself.

The search for the property value can be terminated whenever one
rule has a higher weight than the other rules that apply to the same
element/property combination.

This strategy gives author's style sheets considerably higher
weight than those of the reader. It is therefore important that the
reader has the ability to turn off the influence of a certain style
sheet, e.g. through a pull-down menu.

A declaration in the 'STYLE' attribute of an element (see section 1.1 for an example) has the
same weight as a declaration with an ID-based selector that is
specified at the end of the style sheet:

In the above example, the color of the 'P' element would be red.
Although the specificity is the same for both declarations, the
declaration in the 'STYLE' attribute will override the one in the
'STYLE' element because of cascading rule number 5.

The UA may choose to honor other stylistic HTML attributes, for
example 'ALIGN'. If so, these attributes are translated to the
corresponding CSS rules with specificity equal to 1. The rules are
assumed to be at the start of the author style sheet and may be
overridden by subsequent style sheet rules. In a transition phase,
this policy will make it easier for stylistic attributes to coexist
with style sheets.

CSS1 assumes a simple box-oriented formatting model where each formatted element results in
one or more rectangular boxes. (Elements that have a 'display' value
of 'none' are not formatted and will therefore not result in a box.)
All boxes have a core content area with optional surrounding padding,
border and margin areas.

The size of the margin, border and padding are set with the margin
(5.5.1-5.5.5), padding
(5.5.6-5.5.10), and
border (5.5.11-5.5.22) properties respectively. The padding area
uses the same background as the element itself (set with the background properties (5.3.2-5.3.7)). The color and style for the
border is set with the border properties. The margins are always
transparent, so the parent element will shine through.

The size of the box is the sum of the element width (i.e. formatted
text or image) and the padding, the border and the margin areas.

From the formatter's point of view there are two main types of
elements: block-level and inline.

Elements with a 'display' value of 'block' or 'list-item' are
block-level elements. Also, floating elements (elements with
a 'float' value other than 'none') are considered to be block-level
elements.

The following example shows how margins and padding format a 'UL'
element with two children. To simplify the diagram there are no
borders. Also, the single-letter "constants" in this example are not
legal CSS1 syntax, but is a convenient way to tie the style sheet
values to the figure.

Technically, padding and margin properties are not inherited. But,
as the example shows, the placement of an element is relative to
ancestors and siblings, so these elements' padding and margin
properties have an effect on their children.

If there had been borders in the above example they would have
appeared between the padding and the margins.

The left outer edge is the edge of an element with its
padding, border and margin taken into account. The left inner
edge is the edge of the content only, inside any padding, border
or margin. Ditto for right. The top is the top of the element including any padding,
border and margin; it is only defined for inline and floating
elements, not for non-floating block-level elements. The inner
top is the top of the content, inside any padding, border or
margin. The bottom is the bottom of the element, outside any
padding, border and margin; it is
only defined for inline and floating elements, not for non-floating
block-level elements. The inner bottom is the bottom of the
element, inside any padding, border and margin.

The width of an element is the width of the content, i.e.,
the distance between left inner edge and right inner edge. The
height is the height of the content, i.e., the distance from
inner top to inner bottom.

The width of the margin on non-floating block-level elements
specifies the minimum distance to the edges of surrounding boxes. Two
or more adjoining vertical margins (i.e., with no border, padding or
content between them) are collapsed to use the maximum of the margin
values. In most cases, after collapsing the vertical margins the
result is visually more pleasing and closer to what the designer
expects. In the example above, the margins between the two 'LI'
elements are collapsed by using the maximum of the first LI element's
'margin-bottom' and the second LI element's 'margin-top'. Similarly,
if the padding between the 'UL' and the first 'LI' element (the "E"
constant) had been zero, the margins of the UL and first LI elements
would have been collapsed.

In the case of negative margins, the absolute maximum of the
negative adjoining margins is deducted from the maximum of the
positive adjoining margins. If there are no positive margins, the
absolute maximum of the negative adjoining margins is deducted from
zero.

The horizontal position and size of a non-floating, block-level
element is determined by seven properties: 'margin-left',
'border-left', 'padding-left', 'width', 'padding-right',
'border-right' and 'margin-right'. The sum of these seven is always
equal to the 'width' of the parent element.

By default, the 'width' of an element is 'auto'. If the element is
not a replaced element, this means that the 'width' is calculated by
the UA so that the sum of the seven properties mentioned above is
equal to the parent width. If the element is a replaced element, a
value of 'auto' for 'width' is automatically replaced by the element's
intrinsic width.

Three of the seven properties can be set to 'auto': 'margin-left',
'width' and 'margin-right'. For replaced elements, a value of 'auto'
on 'width' is replaced by the intrinsic width, so for them there can
only be two 'auto' values.

The 'width' has a non-negative UA-defined minimum value (which may
vary from element to element and even depend on other properties). If
'width' goes below this limit, either because it was set explicitly,
or because it was 'auto' and the rules below would make it too small,
the value will be replaced with the minimum value instead.

If exactly one of 'margin-left', 'width' or 'margin-right'
is 'auto', the UA will assign that property a value that will make the
sum of the seven equal to the parent's width.

If none of the properties are 'auto', the value of
'margin-right' will be assigned 'auto'.

If more than one of the three is 'auto', and one of them
is 'width', then the
others ('margin-left' and/or 'margin-right') will be set to zero and
'width' will get the value needed to make the sum of the seven equal
to the parent's width.

Otherwise, if both 'margin-left' and 'margin-right' are 'auto',
they will be set to equal values. This will center the element inside
its parent.

If 'auto' is set as the value for one of the seven properties in an
element that is inline or floating, it will be treated as if it were
set to zero.

Elements with a 'display' property value of 'list-item' are
formatted as block-level elements, but preceded by a list-item marker.
The type of marker is determined by the 'list-style' property. The
marker is placed according to the value of the 'list-style' property:

Using the 'float' property, an element can be
declared to be outside the normal flow of elements and is then
formatted as a block-level element. For example, by setting the
'float' property of an image to 'left', the image is moved to the left
until the margin, padding or border of another block-level element is
reached. The normal flow will wrap around on the right side. The
margins, borders and padding of the element itself will be honored,
and the margins never collapse with the margins of adjacent elements.

A floating element is positioned subject to the following
constraints (see section 4.1 for
an explanation of the terms):

The left outer edge of a left-floating element may not be to the
left of the left inner edge of its parent element. Analogously for
right floating elements.

The left outer edge of a left floating element must be to the
right of the right outer edge of every earlier (in the HTML source)
left-floating element or the top of the former must be lower than the
bottom of the latter. Analogously for right floating elements.

The right outer edge of a left-floating element may not be to the
right of the left outer edge of any right-floating element that is to
the right of it. Analogously for right-floating elements.

A floating element's top may not be higher than the inner top of
its parent.

A floating element's top may not be higher than the top of any
earlier floating or block-level element.

A floating element's top may not be higher than the top of any
line-box (see section 4.4) with content that precedes the
floating element in the HTML source.

A floating element must be placed as high as possible.

A left-floating element must be put as far to the left as
possible, a right-floating element as far to the right as possible. A
higher position is preferred over one that is further to the
left/right.

Elements that are not formatted as block-level elements are
inline elements. An inline element can share line space with
other elements. Consider this example:

<P>Several <EM>emphasized</EM> words <STRONG>appear</STRONG>.</P>

The 'P' element is normally block-level, while 'EM' and 'STRONG'
are inline elements. If the 'P' element is wide enough to format the
whole element on one line, there will be two inline elements on the
line:

Several emphasized words appear.

If there is not enough room on one line an inline element will be
split into several boxes:

<P>Several <EM>emphasized words</EM> appear here.</P>

The above example may be formatted as:

Several emphasizedwords appear here.

If the inline element has margins, borders, padding or text
decorations attached, these will have no effect where the element is
broken:

A replaced element is an element which is replaced by content
pointed to from the element. E.g., in HTML, the 'IMG' element is
replaced by the image pointed to by the 'SRC' attribute. One can
assume that replaced elements come with their own intrinsic
dimensions. If the value of the 'width' property is 'auto', the
intrinsic width is used as the width of the element. If a value other
than 'auto' is specified in the style sheet, this value is used and
the replaced element is resized accordingly (the resize method will
depend on the media type). The 'height' property is used in the same
manner.

All elements have a 'line-height' property that, in principle,
gives the total height of a line of text. Space is added above and
below the text of the line to arrive at that line height. For example,
if the text is 12pt high and 'line-height' is set to '14pt', an extra
space of 2pt is added, namely 1pt above and 1pt below the line. Empty
elements influence these calculations just like elements with content.

The difference between the font size and the 'line-height' is
called the leading. Half the leading is called the
half-leading. After formatting, each line will form a
rectangular line-box.

If a line of text contains sections with different 'line-height'
values (because there are inline elements on the line), then each of
those sections has its own half-leading above and below. The height of
the line-box is from the top of the highest section to the bottom of
the lowest one. Note that the top and bottom do not necessarily
correspond to the tallest element, since elements can be positioned
vertically with the 'vertical-align'
property. To form a paragraph, each line-box is stacked immediately
below the previous line.

Note that any padding, border or margin above and below
non-replaced inline elements does not influence the height of the
line. In other words: if the 'line-height' is too small for the chosen
padding or border, it will overlap with text on other lines.

Replaced elements (e.g. images) on the line can make the line-box
bigger, if the top of the replaced element (i.e., including all of its
padding, border and margin) is above the tallest text section, or if
the bottom is below the lowest.

In the normal case, when there is only one value of 'line-height'
throughout a paragraph, and no tall images, the definition above will
ensure that baselines of successive lines are exactly 'line-height'
apart. This is important when columns of text in different fonts have
to be aligned, for example in a table.

Note that this doesn't preclude the text on two adjacent lines from
overlapping each other. The 'line-height' may be smaller than the
height of the text, in which case the leading will be negative. This
is useful if you know that the text will contain no descenders (e.g.,
because it only contains uppercase), so the lines can be put closer
together.

The canvas is the part of the UA's drawing surface onto which
documents are rendered. No structural element of a document
corresponds to the canvas, and this raises two issues when formatting
a document:

from where should the dimensions of the canvas be set?

when the document doesn't cover the whole canvas, how should this
area be rendered?

A reasonable answer to the first question is that the initial width of the canvas is based on the
window size, but CSS1 leaves this issue for the UA to decide. It is
also reasonable to expect the UA to change the width of the canvas when the
window is resized, but this is also outside the scope of CSS1.

HTML extensions have set a precedent for the second question:
attributes on the 'BODY' element set the background of the whole
canvas. To support designers' expectations, CSS1 introduces a special
rule to find the canvas background:

If the 'background' value of the 'HTML' element is
different from 'transparent' then use it, else use the 'background'
value of the 'BODY' element. If the resulting value is 'transparent',
the rendering is undefined.

The current CSS1 properties and values cannot describe the behavior
of the 'BR' element. In HTML, the 'BR' element specifies a line break
between words. In effect, the element is replaced by a line break.
Future versions of CSS may handle added and replaced content, but
CSS1-based formatters must treat 'BR' specially.

The words between "<" and ">" give a type of value. The most
common types are <length>, <percentage>, <url>,
<number> and <color>; these are described in section 6. The more specialized types (e.g.
<font-family> and <border-style>) are described under the
corresponding property.

Other words are keywords that must appear literally, without
quotes. The slash (/) and the comma (,) must also appear literally.

Several things juxtaposed mean that all of them must occur, in the
given order. A bar (|) separates alternatives: one of them must occur.
A double bar (A || B) means that either A or B or both must occur, in
any order. Brackets ([]) are for grouping. Juxtaposition is stronger
than the double bar, and the double bar is stronger than the bar. Thus
"a b | c || d e" is equivalent to "[ a b ] | [ c || [ d e ]]".

Every type, keyword, or bracketed group may be followed by one of
the following modifiers:

An asterisk (*) indicates that the preceding type, word or group
is repeated zero or more times.

A plus (+) indicates that the preceding type, word or group is
repeated one or more times.

A question mark (?) indicates that the preceding type, word or
group is optional.

A pair of numbers in curly braces ({A,B}) indicates that the
preceding type, word or group is repeated at least A and at most B
times.

Setting font properties will be among the most common uses of style
sheets. Unfortunately, there exists no well-defined and universally
accepted taxonomy for classifying fonts, and terms that apply to one
font family may not be appropriate for others. E.g. 'italic' is
commonly used to label slanted text, but slanted text may also be
labeled as being Oblique, Slanted, Incline, Cursive or
Kursiv. Therefore it is not a simple problem to map typical
font selection properties to a specific font.

Because there is no accepted, universal taxonomy of font
properties, matching of properties to font faces must be done
carefully. The properties are matched in a well-defined order to
insure that the results of this matching process are as consistent as
possible across UAs (assuming that the same library of font faces is
presented to each of them).

The User Agent makes (or accesses) a database of relevant CSS1
properties of all the fonts of which the UA is aware. The UA may be
aware of a font because it has been installed locally or it has been
previously downloaded over the web. If there are two fonts with
exactly the same properties, one of them is ignored.

At a given element and for each character in that element, the UA
assembles the font-properties applicable to that element. Using the
complete set of properties, the UA uses the 'font-family' property to
choose a tentative font family. The remaining properties are tested
against the family according to the matching criteria described with
each property. If there are matches for all the remaining properties,
then that is the matching font face for the given element.

If there is no matching font face within the 'font-family' being
processed by step 2, and if there is a next alternative 'font-family'
in the font set, then repeat step 2 with the next alternative
'font-family'.

If there is a matching font face, but it doesn't contain a glyph
for the current character, and if there is a next alternative
'font-family' in the font sets, then repeat step 2 with the next
alternative 'font-family'. See appendix C
for a description of font and character encoding.

If there is no font within the family selected in 2, then use a
UA-dependent default 'font-family' and repeat step 2, using the best
match that can be obtained within the default font.

(The above algorithm can be optimized to avoid having to revisit
the CSS1 properties for each character.)

The per-property matching rules from (2) above are as follows:

'font-style' is tried first. 'italic'
will be satisfied if there is either a face in the UA's font database
labeled with the CSS keyword 'italic' (preferred) or 'oblique'.
Otherwise the values must be matched exactly or font-style will fail.

'font-variant' is tried next. 'normal'
matches a font not labeled as 'small-caps'; 'small-caps' matches (1) a
font labeled as 'small-caps', (2) a font in which the small caps are
synthesized, or (3) a font where all lowercase letters are replaced by
upper case letters. A small-caps font may be synthesized by
electronically scaling uppercase letters from a normal font.

'font-weight' is matched next, it will
never fail. (See 'font-weight' below.)

'font-size' must be matched within a
UA-dependent margin of tolerance. (Typically, sizes for scalable fonts
are rounded to the nearest whole pixel, while the tolerance for
bitmapped fonts could be as large as 20%.) Further computations, e.g.
by 'em' values in other properties, are based on the 'font-size' value
that is used, not the one that is specified.

The 'font-style' property selects between normal (sometimes
referred to as "roman" or "upright"), italic and oblique faces within
a font family.

A value of 'normal' selects a font that is classified as 'normal'
in the UA's font database, while 'oblique' selects a font that is
labeled 'oblique'. A value of 'italic' selects a font that is labeled
'italic', or, if that is not available, one labeled 'oblique'.

The font that is labeled 'oblique' in the UA's font database may
actually have been generated by electronically slanting a normal font.

Fonts with Oblique, Slanted or Incline in their names will
typically be labeled 'oblique' in the UA's font database. Fonts with
Italic, Cursive or Kursiv in their names will
typically be labeled 'italic'.

H1, H2, H3 { font-style: italic }
H1 EM { font-style: normal }

In the example above, emphasized text within 'H1' will appear in a
normal face.

Another type of variation within a font family is the small-caps.
In a small-caps font the lower case letters look similar to the
uppercase ones, but in a smaller size and with slightly different
proportions. The 'font-variant' property selects that font.

A value of 'normal' selects a font that is not a small-caps font,
'small-caps' selects a small-caps font. It is acceptable (but not
required) in CSS1 if the small-caps font is a created by taking a
normal font and replacing the lower case letters by scaled uppercase
characters. As a last resort, uppercase letters will be used as
replacement for a small-caps font.

The following example results in an 'H3' element in small-caps,
with emphasized words in oblique small-caps:

H3 { font-variant: small-caps }
EM { font-style: oblique }

There may be other variants in the font family as well, such as
fonts with old-style numerals, small-caps numerals, condensed or
expanded letters, etc. CSS1 has no properties that select those.

CSS1 core: insofar as this property causes text to be
transformed to uppercase, the same considerations as for 'text-transform' apply.

The 'font-weight' property selects the weight of the font. The
values '100' to '900' form an ordered sequence, where each number
indicates a weight that is at least as dark as its predecessor. The
keyword 'normal' is synonymous with '400', and 'bold' is synonymous
with '700'. Keywords other than 'normal' and 'bold' have been shown to
be often confused with font names and a numerical scale was therefore
chosen for the 9-value list.

The 'bolder' and 'lighter' values select font weights that are
relative to the weight inherited from the parent:

STRONG { font-weight: bolder }

Child elements inherit the resultant weight, not the keyword value.

Fonts (the font data) typically have one or more properties whose
values are names that are descriptive of the "weight" of a font. There
is no accepted, universal meaning to these weight names. Their primary
role is to distinguish faces of differing darkness within a single
font family. Usage across font families is quite variant; for example
a font that you might think of as being bold might be described as
being Regular, Roman, Book, Medium, Semi- or DemiBold,
Bold, or Black, depending on how black the "normal" face
of the font is within the design. Because there is no standard usage
of names, the weight property values in CSS1 are given on a numerical
scale in which the value '400' (or 'normal') corresponds to the
"normal" text face for that family. The weight name associated with
that face will typically be Book, Regular, Roman, Normal or
sometimes Medium.

The association of other weights within a family to the numerical
weight values is intended only to preserve the ordering of darkness
within that family. However, the following heuristics tell how the
assignment is done in typical cases:

If the font family already uses a numerical scale with nine values
(like e.g. OpenType does), the font weights should be mapped
directly.

If there is both a face labeled Medium and one labeled
Book, Regular, Roman or Normal, then the
Medium is normally assigned to the '500'.

The font labeled "Bold" will often correspond to the weight value '700'.

If there are fewer then 9 weights in the family, the default
algorithm for filling the "holes" is as follows. If '500' is
unassigned, it will be assigned the same font as '400'. If any of the
values '600', '700', '800' or '900' remains unassigned, they are
assigned to the same face as the next darker assigned keyword, if any,
or the next lighter one otherwise. If any of '300', '200' or '100'
remains unassigned, it is assigned to the next lighter assigned
keyword, if any, or the next darker otherwise.

The following two examples illustrate the process. Assume four
weights in the "Example1" family, from lightest to darkest:
Regular, Medium, Bold, Heavy. And assume six weights in the
"Example2" family: Book, Medium, Bold, Heavy, Black,
ExtraBlack. Note how in the second example it has been decided
not to assign "Example2 ExtraBlack" to anything.

Since the intent of the relative keywords 'bolder' and 'lighter' is
to darken or lighten the face within the family and because a
family may not have faces aligned with all the symbolic weight values,
the matching of 'bolder' is to the next darker face available on the
client within the family and the matching of 'lighter' is to the next
lighter face within the family. To be precise, the meaning of the
relative keywords 'bolder' and 'lighter' is as follows:

'bolder' selects the next weight that is assigned to a font that
is darker than the inherited one. If there is no such weight, it
simply results in the next darker numerical value (and the font
remains unchanged), unless the inherited value was '900' in which case
the resulting weight is also '900'.

'lighter' is similar, but works in the opposite direction: it
selects the next lighter keyword with a different font from the
inherited one, unless there is no such font, in which case it selects
the next lighter numerical value (and keeps the font unchanged).

There is no guarantee that there will be a darker face for each of
the 'font-weight' values; for example, some fonts may have only a
normal and a bold face, others may have eight different face weights.
There is no guarantee on how a UA will map font faces within a family
to weight values. The only guarantee is that a face of a given value
will be no less dark than the faces of lighter values.

An <absolute-size> keyword is an index to a table of font
sizes computed and kept by the UA. Possible values are: [ xx-small |
x-small | small | medium | large | x-large | xx-large ]. On a computer
screen a scaling factor of 1.5 is suggested between adjacent indexes;
if the 'medium' font is 10pt, the 'large' font could be 15pt.
Different media may need different scaling factors. Also, the UA
should take the quality and availability of fonts into account when
computing the table. The table may be different from one font family
to another.

<relative-size>

A <relative-size> keyword is interpreted relative to the
table of font sizes and the font size of the parent element. Possible
values are: [ larger | smaller ]. For example, if the parent element
has a font size of 'medium', a value of 'larger' will make the font
size of the current element be 'large'. If the parent element's size
is not close to a table entry, the UA is free to interpolate between
table entries or round off to the closest one. The UA may have to
extrapolate table values if the numerical value goes beyond the
keywords.

Length and percentage values should not take the font size table
into account when calculating the font size of the element.

Negative values are not allowed.

On all other properties, 'em' and 'ex' length values refer to the
font size of the current element. On the 'font-size' property, these
length units refer to the font size of the parent element.

Note that an application may reinterpret an explicit size,
depending on the context. E.g., inside a VR scene a font may get a
different size because of perspective distortion.

In the second rule, the font size percentage value ('80%') refers
to the font size of the parent element. In the third rule, the line
height percentage refers to the font size of the element itself.

In the first three rules above, the 'font-style', 'font-variant'
and 'font-weight' are not explicitly mentioned, which means they are
all three set to their initial value ('normal'). The fourth rule sets
the 'font-weight' to 'bold', the 'font-style' to 'italic' and
implicitly sets 'font-variant' to 'normal'.

The fifth rule sets the 'font-variant' ('small-caps'), the
'font-size' (120% of the parent's font), the 'line-height' (120% times
the font size) and the 'font-family' ('fantasy'). It follows that the
keyword 'normal' applies to the two remaining properties: 'font-style'
and 'font-weight'.

These properties describe the color (often called foreground
color) and background of an element (i.e. the surface onto which
the content is rendered). One can set a background color and/or a
background image. The position of the image, if/how it is repeated,
and whether it is fixed or scrolled relative to the canvas can also be
set.

The 'color' property inherits normally. The background properties
do not inherit, but the parent element's background will shine through
by default because of the initial 'transparent' value on
'background-color'.

This property sets the background image of an element. When setting
a background image, one should also set a background color that will
be used when the image is unavailable. When the image is available, it
is overlaid on top of the background color.

If a background image is specified, the value of
'background-repeat' determines how/if the image is repeated.

A value of 'repeat' means that the image is repeated both
horizontally and vertically. The 'repeat-x' ('repeat-y') value makes
the image repeat horizontally (vertically), to create a single band of
images from one side to the other. With a value of 'no-repeat', the
image is not repeated.

CSS1 core: UAs may treat 'fixed' as 'scroll'. However, it
is recommended they interpret 'fixed' correctly, at least on the HTML
and BODY elements, since there is no way for an author to provide an
image only for those browsers that support 'fixed'. (See section 7.)

If a background image has been specified, the value of
'background-position' specifies its initial position.

With a value pair of '0% 0%', the upper left corner of the image is
placed in the upper left corner of the box that surrounds the content
of the element (i.e., not the box that surrounds the padding, border
or margin). A value pair of '100% 100%' places the lower right corner
of the image in the lower right corner of the element. With a value
pair of '14% 84%', the point 14% across and 84% down the image is to
be placed at the point 14% across and 84% down the element.

With a value pair of '2cm 2cm', the upper left corner of the image
is placed 2cm to the right and 2cm below the upper left corner of the
element.

If only one percentage or length value is given, it sets the
horizontal position only, the vertical position will be 50%. If two
values are given, the horizontal position comes first. Combinations of
length and percentage values are allowed, e.g. '50% 2cm'. Negative
positions are allowed.

One can also use keyword values to indicate the position of the
background image. Keywords cannot be combined with percentage values,
or length values. The possible combinations of keywords and their
interpretations are as follows:

'top left' and 'left top' both mean the same as '0% 0%'.

'top', 'top center' and 'center top' mean the same as '50% 0%'.

'right top' and 'top right' mean the same as '100% 0%'.

'left', 'left center' and 'center left' mean the same as '0% 50%'.

'center' and 'center center' mean the same as '50% 50%'.

'right', 'right center' and 'center right' mean the same as '100% 50%'.

'bottom left' and 'left bottom' mean the same as '0% 100%'.

'bottom', 'bottom center' and 'center bottom' mean the same as '50% 100%'.

The 'background' property is a shorthand property for setting the
individual background properties (i.e., 'background-color',
'background-image', 'background-repeat', 'background-attachment' and
'background-position') at the same place in the style sheet.

Possible values on the 'background' properties are the set of all
possible values on the individual properties.

The 'background' property always sets all the individual background
properties. In the first rule of the above example, only a value for
'background-color' has been given and the other individual properties
are set to their initial value. In the second rule, all individual
properties have been specified.

The length unit indicates an addition to the default space between
words. Values can be negative, but there may be
implementation-specific limits. The UA is free to select the exact
spacing algorithm. The word spacing may also be influenced by
justification (which is a value of the 'text-align' property).

H1 { word-spacing: 1em }

Here, the word-spacing between each word in 'H1' elements would be
increased by '1em'.

The length unit indicates an addition to the default space between
characters. Values can be negative, but there may be
implementation-specific limits. The UA is free to select the exact
spacing algorithm. The letter spacing may also be influenced by
justification (which is a value of the 'align' property).

BLOCKQUOTE { letter-spacing: 0.1em }

Here, the letter-spacing between each character in 'BLOCKQUOTE'
elements would be increased by '0.1em'.

With a value of 'normal', the UAs may change the space between
letters to justify text. This will not happen if 'letter-spacing' is
explicitly set to a <length> value:

BLOCKQUOTE { letter-spacing: 0 }
BLOCKQUOTE { letter-spacing: 0cm }

When the resultant space between two letters is not the same as the
default space, UAs should not use ligatures.

This property describes decorations that are added to the text of
an element. If the element has no text (e.g. the 'IMG' element in
HTML) or is an empty element (e.g. '<EM></EM>'), this
property has no effect. A value of 'blink' causes the text to blink.

The color(s) required for the text decoration should be derived
from the 'color' property value.

This property is not inherited, but elements should match their
parent. E.g., if an element is underlined, the line should span the
child elements. The color of the underlining will remain the same even
if descendant elements have different 'color' values.

A:link, A:visited, A:active { text-decoration: underline }

The example above would underline the text of all links (i.e., all
'A' elements with a 'HREF' attribute).

UAs must recognize the keyword 'blink', but are not required to
support the blink effect.

The property affects the vertical positioning of the element. One
set of keywords is relative to the parent element:

'baseline'

align the baseline of the element (or the bottom, if the element
doesn't have a baseline) with the baseline of the parent

'middle'

align the vertical midpoint of the element (typically an image)
with the baseline plus half the x-height of the parent

'sub'

subscript the element

'super'

superscript the element

'text-top'

align the top of the element with the top of the parent element's
font

'text-bottom'

align the bottom of the element with the bottom of the parent
element's font

Another set of properties are relative to the formatted line that
the element is a part of:

'top'

align the top of the element with the tallest element on the line

'bottom'

align the bottom of the element with the lowest element on the
line

Using the 'top' and 'bottom' alignment, unsolvable situations can
occur where element dependencies form a loop.

Percentage values refer to the value of the 'line-height' property
of the element itself. They raise the baseline of the element (or the
bottom, if it has no baseline) the specified amount above the baseline
of the parent. Negative values are possible. E.g., a value of '-100%'
will lower the element so that the baseline of the element ends up
where the baseline of the next line should have been. This allows
precise control over the vertical position of elements (such as images
that are used in place of letters) that don't have a baseline.

It is expected that a future version of CSS will allow <length> as a value on
this property.

The actual transformation in each case is human language dependent.
See [4] for ways to find the language of an
element.

H1 { text-transform: uppercase }

The example above would put 'H1' elements in uppercase text.

CSS1 core: UAs may ignore 'text-transform' (i.e., treat it
as 'none') for characters that are not from the Latin-1 repertoire and
for elements in languages for which the transformation is different
from that specified by the case-conversion tables of Unicode [8].

This property describes how text is aligned within the element. The
actual justification algorithm used is UA and human language
dependent.

Example:

DIV.center { text-align: center }

Since 'text-align' inherits, all block-level elements inside the
'DIV' element with 'CLASS=center' will be centered. Note that
alignments are relative to the width of the element, not the canvas.
If 'justify' is not supported, the UA will supply a replacement.
Typically, this will be 'left' for western languages.

CSS1 core: UAs may treat 'justify' as 'left' or 'right',
depending on whether the element's default writing direction is
left-to-right or right-to-left, respectively.

The property specifies the indentation that appears before the
first formatted line. The value of 'text-indent' may be negative, but
there may be implementation-specific limits. An indentation is not
inserted in the middle of an element that was broken by another (such
as 'BR' in HTML).

When a numerical value is specified, the line height is given by
the font size of the current element multiplied with the numerical
value. This differs from a percentage value in the way it inherits:
when a numerical value is specified, child elements will inherit the
factor itself, not the resultant value (as is the case with percentage and other units).

Negative values are not allowed.

The three rules in the example below have the same resultant line
height:

The box properties set the size, circumference and position of the
boxes that represent elements. See the formatting model (section 4) for examples
on how to use the box properties.

The margin properties set the margin
of an element. The 'margin' property sets the
margin for all four sides while the other margin properties
only set their respective side.

The padding properties describe how much space to insert between
the border and the content (e.g., text or image). The 'padding'
property sets the padding for all four sides while the other padding
properties only set their respective side.

The border properties set the borders of an element. Each element
has four borders, one on each side, that are defined by their width,
color and style.

The 'width' and 'height' properties set the size of the box, and
the 'float' and 'clear' properties can alter the position of elements.

The 'margin' property is a shorthand property for setting
'margin-top', 'margin-right', 'margin-bottom' and 'margin-left' at the
same place in the style sheet.

If four length values are specified they apply to top, right,
bottom and left respectively. If there is only one value, it applies
to all sides, if there are two or three, the missing values are taken
from the opposite side.

The 'padding' property is a shorthand property for setting
'padding-top', 'padding-right', 'padding-bottom' and 'padding-left' at
the same place in the style sheet.

If four values are specified they apply to top, right, bottom and
left respectively. If there is only one value, it applies to all
sides, if there are two or three, the missing values are taken from
the opposite side.

The surface of the padding area is set with the 'background'
property:

H1 {
background: white;
padding: 1em 2em;
}

The example above sets a '1em' padding vertically ('padding-top'
and 'padding-bottom') and a '2em' padding horizontally
('padding-right' and 'padding-left'). The 'em' unit is relative to the
element's font size: '1em' is equal to the size of the font in use.

The 'border' property is a shorthand property for setting the same
width, color and style on all four borders of an element. For example,
the first rule below is equivalent to the set of four rules shown
after it:

Unlike the shorthand 'margin' and 'padding' properties, the
'border' property cannot set different values on the four borders. To
do so, one or more of the other border properties must be used.

Since the properties to some extent have overlapping functionality,
the order in which the rules are specified becomes important. Consider
this example:

BLOCKQUOTE {
border-color: red;
border-left: double
color: black;
}

In the above example, the color of the left border will be black,
while the other borders are red. This is due to 'border-left' setting
the width, style and color. Since the color value is not specified on
the 'border-left' property, it will be taken from the 'color'
property. The fact that the 'color' property is set after the
'border-left' property is not relevant.

Note that while the 'border-width' property accepts up to four
length values, this property only accepts one.

This property can be applied to text elements, but it is most
useful with replaced elements such as images. The width is to be
enforced by scaling the image if necessary. When scaling, the aspect
ratio of the image is preserved if the 'height' property is 'auto'.

Example:

IMG.icon { width: 100px }

If the 'width' and 'height' of a replaced element are both 'auto',
these properties will be set to the intrinsic dimensions of the
element.

This property can be applied to text, but it is most useful with
replaced elements such as images. The height is to be enforced by
scaling the image if necessary. When scaling, the aspect ratio of the
image is preserved if the 'width' property is 'auto'.

Example:

IMG.icon { height: 100px }

If the 'width' and 'height' of a replaced element are both 'auto',
these properties will be set to the intrinsic dimensions of the
element.

If applied to a textual element, the height can be enforced with
e.g. a scrollbar.

Negative values are not allowed.

CSS1 core: UAs may ignore the 'height' property (i.e.,
treat it as 'auto') if the element is not a replaced element.

With the value 'none', the element will be displayed where it
appears in the text. With a value of 'left' ('right') the element will
be moved to the left (right) and
the text will wrap on the right (left) side of the element. With a
value of 'left' or 'right', the element is treated as block-level
(i.e. the 'display' property is ignored). See section 4.1.4 for a full specification.

IMG.icon {
float: left;
margin-left: 0;
}

The above example will place all IMG elements with 'CLASS=icon'
along the left side of the parent element.

This property is most often used with inline images, but also
applies to text elements.

This property specifies if an element allows floating elements on
its sides. More specifically, the value of this property lists the
sides where floating elements are not accepted. With 'clear' set to
'left', an element will be moved below any floating element on the
left side. With 'clear' set to 'none', floating elements are allowed
on all sides. Example:

These properties classify elements into categories more than they
set specific visual parameters.

The list-style properties describe how list items (i.e. elements
with a 'display' value of 'list-item') are formatted. The list-style
properties can be set on any element, and it will inherit normally
down the tree. However, they will only be have effect on elements with
a 'display' value of 'list-item'. In HTML this is typically the case
for the 'LI' element.

This property describes how/if an element is displayed on the
canvas (which may be on a printed page, a computer display etc.).

An element with a 'display' value of 'block' opens a new box. The
box is positioned relative to adjacent boxes according to the CSS formatting model. Typically, elements
like 'H1' and 'P' are of type 'block'. A value of 'list-item' is
similar to 'block' except that a list-item marker is added. In HTML,
'LI' will typically have this value.

An element with a 'display' value of 'inline' results in a new
inline box on the same line as the previous content. The box is
dimensioned according to the formatted size of the content. If the
content is text, it may span several lines, and there will be a box on
each line. The margin, border and padding properties apply to 'inline'
elements, but will not have any effect at the line breaks.

A value of 'none' turns off the display of the element, including
children elements and the surrounding box.

This property declares how whitespace inside the element is
handled: the 'normal' way (where whitespace is collapsed), as 'pre'
(which behaves like the 'PRE' element in HTML) or as 'nowrap' (where
wrapping is done only through BR elements):

PRE { white-space: pre }
P { white-space: normal }

The initial value of 'white-space' is 'normal', but a UA will
typically have default values for all HTML elements according to the
suggested rendering of elements in the HTML specification [2].

Since the specificity (as defined in the cascading order) is higher for the first
rule in the style sheet in the example above, it will override the
second rule on all 'LI' elements and only 'lower-alpha' list styles
will be used. It is therefore recommended to set 'list-style' only on
the list type elements:

OL.alpha { list-style: lower-alpha }
UL { list-style: disc }

In the above example, inheritance will transfer the 'list-style' values from
'OL' and 'UL' elements to 'LI' elements.

A URL value can be combined with any other value:

UL { list-style: url(http://png.com/ellipse.png) disc }

In the example above, the 'disc' will be used when the image is unavailable.

The format of a length value is an optional sign character ('+' or
'-', with '+' being the default) immediately followed by a number
(with or without a decimal point) immediately followed by a unit
identifier (a two-letter abbreviation). After a '0' number, the unit
identifier is optional.

Some properties allow negative length units, but this may
complicate the formatting model and there may be
implementation-specific limits. If a negative length value cannot be
supported, it should be clipped to the nearest value that can be
supported.

There are two types of length units: relative and absolute.
Relative units specify a length relative to another length property.
Style sheets that use relative units will more easily scale from one
medium to another (e.g. from a computer display to a laser printer).
Percentage units (described below) and
keyword values (e.g. 'x-large') offer similar advantages.

The relative units 'em' and 'ex' are relative to the font size of
the element itself. The only exception to this rule in CSS1 is the
'font-size' property where 'em' and 'ex' values refer to the font size
of the parent element.

Pixel units, as used in the last rule, are relative to the
resolution of the canvas, i.e. most often a computer display. If the
pixel density of the output device is very different from that of a
typical computer display, the UA should rescale pixel values. The
suggested reference pixel is the visual angle of one pixel on
a device with a pixel density of 90dpi and a distance from the reader
of an arm's length. For a nominal arm's length of 28 inches, the
visual angle is about 0.0227 degrees.

The format of a percentage value is an optional sign character ('+' or '-',
with '+' being the default) immediately followed by a number (with or without
a decimal point) immediately followed by '%'.

Percentage values are always relative to another value, for example a length
unit. Each property that allows percentage units also defines what value
the percentage value refer to. Most often this is the font size of the element
itself:

P { line-height: 120% } /* 120% of the element's 'font-size' */

In all inherited CSS1 properties, if the value is specified as a
percentage, child elements inherit the resultant value, not the
percentage value.

The format of an RGB value in hexadecimal notation is a '#'
immediately followed by either three or six hexadecimal characters.
The three-digit RGB notation (#rgb) is converted into six-digit form
(#rrggbb) by replicating digits, not by adding zeros. For example,
#fb0 expands to #ffbb00. This makes sure that white (#ffffff) can be
specified with the short notation (#fff) and removes any dependencies
on the color depth of the display.

The format of an RGB value in the functional notation is 'rgb('
followed by a comma-separated list of three numerical values (either
three integer values in the range of 0-255, or three percentage values
in the range of 0.0% to 100.0%) followed by ')'. Whitespace characters
are allowed around the numerical values.

Values outside the numerical ranges should be clipped. The three
rules below are therefore equivalent:

RGB colors are specified in the sRGB color space [9]. UAs may vary in the fidelity with which they
represent these colors, but use of sRGB provides an unambiguous and
objectively measurable definition of what the color should be, which
can be related to international standards [10].

UAs may limit their efforts in displaying colors to performing a
gamma-correction on them. sRGB specifies a display gamma of 2.2 under
specified viewing conditions. UAs adjust the colors given in CSS such
that, in combination with an output device's "natural" display gamma,
an effective display gamma of 2.2 is produced. Appendix D gives further details of this. Note
that only colors specified in CSS are affected; e.g., images are
expected to carry their own color information.

A Uniform Resource Locator (URL) is identified with a functional
notation:

BODY { background: url(http://www.bg.com/pinkish.gif) }

The format of a URL value is 'url(' followed by optional white space followed
by an optional single quote (') or double quote (") character followed by
the URL itself (as defined in [11]) followed by an optional
single quote (') or double quote (") character followed by optional whitespace
followed by ')'. Quote characters that are not part of the URL itself must
be balanced.

Parentheses, commas, whitespace characters, single quotes (') and double
quotes (") appearing in a URL must be escaped with a backslash: '\(', '\)',
'\,'.

Partial URLs are interpreted relative to the source of the style sheet, not
relative to the document:

A User Agent that uses CSS1 to display documents conforms to the CSS1
specification if it:

attempts to fetch all referenced style sheets and parse them
according to this specification

sorts the declarations according to the cascading order

implements the CSS1 functionality within the constraints of the
presentation medium (see explanation below).

A User Agent that outputs CSS1 style sheets conforms to the CSS1
specification if it:

outputs valid CSS1 style sheets

A User Agent that uses CSS1 to display documents and
outputs CSS1 style sheets conforms to the CSS1 specification if it
meets both sets of conformance requirements.

A UA does not have to implement all the functionality of CSS1: it
can conform to CSS1 by implementing the core functionality. The core
functionality consists of the whole CSS1 specification except those
parts explicitly excluded. In the text, those parts are marked with
"CSS1 core:" followed by an explanation of what functionality
is outside the core functionality. The set of features excluded from
the core functionality is called CSS1 advanced features.

This section only defines conformance to CSS1. There will be other
levels of CSS in the future that may require a UA to implement a
different set of features in order to conform.

Examples of constraints of the presentation medium are: limited
resources (fonts, color) and limited resolution (so margins may not be
accurate). In these cases, the UA should approximate the style sheet
values. Also, different user interface paradigms may have their own
constraints: a VR browser may rescale the document based on its
"distance" from the user.

UAs may offer readers additional choices on presentation. For
example, the UA may provide options for readers with visual
impairments or may provide the choice to disable blinking.

Note that CSS1 does not specify all aspects of formatting. E.g.,
the UA is free to select a letter-spacing algorithm.

This specification also recommends, but doesn't require, that a UA:

allows the reader to specify personal style sheets

allows individual style sheets to be turned on and off

The above conformance rules describe only functionality, not user
interface.

This specification defines CSS level 1. It is expected that higher
levels of CSS, with additional features, will be defined in the
future. To ensure that UAs supporting just CSS1 will be able to read
style sheets containing higher level features, this section defines
what the UA does when it encounters certain constructs that are not
valid in CSS level 1.

a declaration with an unknown property is ignored. For example, if
the style sheet is

H1 { color: red; rotation: 70deg }

the UA will treat this as if the style sheet had been

H1 { color: red; }

illegal values, or values with illegal parts, are treated
as if the declaration weren't there at all:

In the above example, a CSS1 parser would honor the first rule and ignore
the rest, as if the style sheet had been

IMG { float: left }
IMG { }
IMG { }
IMG { }

A UA conforming to a future CSS specification may accept one or
more of the other rules as well.

an invalid at-keyword is ignored together with everything
following it, up to and including the next semicolon (;) or brace pair
({...}), whichever comes first. For example, assume the style sheet
reads:

The '@three-dee' is illegal according to CSS1. Therefore, the whole at-rule
(up to, and including, the third right curly brace) is ignored. The CSS1
UA skips it, effectively reducing the style sheet to:

H1 {color: blue}

In more detail:

A CSS style sheet, for any version of CSS, consists of a list of
statements. There are two kinds of statements:
at-rules and rulesets. There may be whitespace
(spaces, tabs, newlines) around the statements.

CSS style sheets are often embedded in HTML documents, and to be able
to hide style sheets from older UAs, it is convenient put the style
sheets inside HTML comments. The HTML comment tokens "<!--" and
"-->" may occur before, after, and in between the statements. They
may have whitespace around them.

At-rules start with an at-keyword, which is an
identifier with an '@' at the start (for example: '@import', '@page').
An identifier consists of letters, digits, dashes and escaped
characters (defined below).

An at-rule consists of everything up to and including the next
semicolon (;) or the next block (defined shortly), whichever comes
first. A CSS1 UA that encounters an at-rule that starts with an
at-keyword other than '@import' ignores the whole of the at-rule and
continue parsing after it. It also ignores any at-rule that starts
with '@import' if it doesn't occur at the top of the style sheet,
i.e., if it occurs after any rules (even ignored rules). Here is an
example.

Assume a CSS1 parser encounters this style sheet:

@import "subs.css";
H1 { color: blue }
@import "list.css";

The second '@import' is illegal according to CSS1. The CSS1 parser
skips the whole at-rule, effectively reducing the style sheet to:

@import "subs.css";
H1 {color: blue}

A block starts with a left curly brace ({) and ends with
the matching right curly brace (}). In between there may be any
characters, except that parentheses (()), brackets ([]) and braces
({}) always occur in matching pairs and may be nested. Single (') and
double quotes (") also occur in matching pairs, and characters between
them are parsed as a string (see the tokenizer in appendix B for a definition of string). Here is
an example of a block; note that the right brace between the quotes
does not match the opening brace of the block, and that the second
single quote is an escaped character, and thus doesn't match the
opening quote:

{ causta: "}" + ({7} * '\'') }

A ruleset consists of a selector-string followed by a
declaration-block. The selector-string consists of
everything up to (but not including) the first left curly brace ({). A
ruleset that starts with a selector-string that is not valid CSS1 is
skipped.

The second line contains a selector-string that is illegal in CSS1. The CSS1
UA will skip the ruleset, reducing the style sheet to:

H1 { color: blue }
P EM { font-weight: bold }

A declaration-block starts with a left curly brace ({) and ends with the
matching right curly brace (}). In between there is a list of zero or more
declarations, separated by semicolons (;).

A declaration consists of a property, a colon (:) and a
value. Around each of these there may be whitespace. A property
is an identifier, as defined earlier. Any character may occur in the value,
but parentheses (()), brackets ([]), braces ({}), single quotes (') and double
quotes (") must come in matching pairs. Parentheses, brackets, and braces
may be nested. Inside the quotes, characters are parsed as a string.

To ensure that new properties and new values for existing
properties can be added in the future, a UA must skip a declaration
with an invalid property name or an invalid value. Every CSS1 property
has its own syntactic and semantic restrictions on the values it
accepts.

The second declaration on the first line has an invalid value '12pt'. The
second declaration on the second line contains an undefined property
'font-vendor'. The CSS1 parser will skip these declarations, reducing the
style sheet to:

Comments (see section 1.7) can occur
anywhere where whitespace can occur and are considered to be
whitespace. CSS1 defines additional places where whitespace can occur
(such as inside values) and comments are allowed there as well.

The following rules always hold:

All CSS style sheets are case-insensitive, except for parts that
are not under the control of CSS. I.e., in CSS1, font family names and
URLs can be case-sensitive. Also, the case-sensitivity of the CLASS
and ID attributes is under the control of HTML [2].

in CSS1, selectors (element names, classes and IDs) can contain only the
characters A-Z, 0-9, and Unicode characters 161-255, plus dash (-); they
cannot start with a dash or a digit; they can also contain escaped characters
and any Unicode character as a numeric code (see next item).

the backslash followed by at most four hexadecimal digits (0..9A..F) stands
for the Unicode character with that number.

any character except a hexadecimal digit can be escaped to remove its special meaning, by putting a backslash in front, Example: "\"" is a string consisting of one double quote.

the two preceding items define backslash-escapes. Backslash-escapes are always considered to be part of an identifier, except inside strings (i.e., "\7B" is not punctuation, even though "{" is, and "\32" is allowed at the start of a class name, even though "2" is not).

Note: The CLASS attribute of HTML allows more characters in a class
name than the set allowed for selectors above. In CSS1, these
characters have to be escaped or written as Unicode numbers:
"B&W?" can be written as "B\&W\?" or "B\26W\3F", "κουρος"
(Greek: "kouros") has to be written as "\3BA\3BF\3C5\3C1\3BF\3C2". It
is expected that in later versions of CSS more characters can be
entered directly.

Three people deserve special mentioning: Dave Raggett (for his
encouragement and work on HTML3), Chris Lilley (for his continued
contributions, especially in the area of colors and fonts) and Steven
Pemberton (for his organizational as well as creative skills).

The following style sheet is written according to the suggested
rendering in the HTML 2.0 [3] specification. Some
styles, e.g. colors, have been added for completeness. It is suggested
that a style sheet similar to the one below is used as the UA default.

The minimal CSS (i.e., any version of CSS) grammar that all
implementations need to support is defined in section 7. The grammar below defines a
much smaller language, a language that defines the syntax of CSS1.

It is in some sense, however, still a superset of CSS1: there are
additional semantic constraints not expressed in this grammar. A
conforming UA must also adhere to the forward-compatible parsing rules
(section 7.1), the property and value notation (section 5) and the
unit notation (section 6). In addition, HTML imposes restrictions,
e.g., on the possible values of the CLASS attribute.

The grammar below is LL(1) (but note that most UA's should not use
it directly, since it doesn't express the parsing conventions, only
the CSS1 syntax). The format of the productions is optimized for human
consumption and some shorthand notation beyond yacc [15] is used:

HTML documents may contain any of the about 30,000 different
characters defined by Unicode. Many documents only need a few hundred.
Many fonts also only contain just a few hundred glyphs. In combination
with section 5.2, this appendix
explains how the characters in the document and the glyphs in a font
are matched.

The content of an HTML document is a sequence of
characters and markup. To send it "over the wire", it is
encoded as a sequence of bytes, using one of several possible
encodings. The HTML document has to be decoded to find the
characters. For example, in Western Europe it is customary to use the
byte 224 for an a-with-grave-accent (à), but in Hebrew, it is
more common to use 224 for an Aleph. In Japanese, the meaning of a
byte usually depends on the bytes that preceded it. In some encodings,
one character is encoded as two (or more) bytes.

The UA knows how to decode the bytes by looking at the "charset"
parameter in the HTTP header. Typical encodings (charset values) are
"ASCII" (for English), "ISO-8859-1" (for Western Europe), "ISO-8859-8"
(for Hebrew), "Shift-JIS" (for Japanese).

HTML [2][4], allows some
30,000 different characters, namely those defined by Unicode. Not many
documents will use that many different characters, and choosing the
right encoding will usually ensure that the document only needs one
byte per character. Occasional characters outside the encoded range
can still be entered as numerical character references: '&#928;'
will always mean the Greek uppercase Pi, no matter what encoding was
used. Note that this entails that UAs have to be prepared for any
Unicode character, even if they only handle a few encodings.

A font doesn't contain characters, it contains pictures of
characters, known as glyphs. The glyphs, in the form of
outlines or bitmaps, constitute a particular representation of a
character. Either explicitly or implicitly, each font has a table
associated with it, the font encoding table, that tells for
each glyph what character it is a representation for. In Type 1 fonts,
the table is referred to as an encoding vector.

In fact, many fonts contain several glyphs for the same character.
Which of those glyphs should be used depends either on the rules of
the language, or on the preference of the designer.

In Arabic, for example, all letters have four different shapes,
depending on whether the letter is used at the start of a word, in the
middle, at the end, or in isolation. It is the same character in all
cases, and thus there is only one character in the HTML document, but
when printed, it looks differently each time.

There are also fonts that leave it to the graphic designer to choose from
among various alternative shapes provided. Unfortunately, CSS1 doesn't yet
provide the means to select those alternatives. Currently, it is always the
default shape that is chosen from such fonts.

To deal with the problem that a single font may not be enough to display
all the characters in a document, or even a single element, CSS1 allows the
use of font sets.

A font set in CSS1 is a list of fonts, all of the same style and size, that
are tried in sequence to see if they contain a glyph for a certain character.
An element that contains English text mixed with mathematical symbols may
need a font set of two fonts, one containing letters and digits, the other
containing mathematical symbols. See section 5.2
for a detailed description of the selection mechanism for font sets.

Here is an example of a font set suitable for a text that is expected to
contain text with Latin characters, Japanese characters, and mathematical
symbols:

BODY { font-family: Baskerville, Mincho, Symbol, serif }

The characters available in the Baskerville font (a font with only Latin
characters) will be taken from that font, Japanese will be taken from Mincho,
and the mathematical symbols will come from Symbol. Any other characters
will (hopefully) come from the generic font family 'serif'. The 'serif' font
family will also be used if one or more of the other fonts is unavailable.

See the Gamma
Tutorial in the PNG specification [12] if you aren't
already familiar with gamma issues.

In the computation, UAs displaying on a CRT may assume an ideal CRT
and ignore any effects on apparent gamma caused by dithering. That
means the minimal handling they need to do on current platforms is:

The goal of the work on CSS1 has been to create a simple style
sheet mechanism for HTML documents. The current specification is a
balance between the simplicity needed to realize style sheets on the
web, and pressure from authors for richer visual control. CSS1 offers:

nicer markup: instead of using "FONT" elements to achieve the
popular small-caps style, one declaration in the style sheet is
sufficient. Compare the visual markup:

<H1>H<FONT SIZE=-1>EADLINE</FONT></H1>

with the style sheet:

H1 { font-style: small-caps }
<H1>Headline</H1>

various integration levels: CSS1 style rules can be fetched from
external style sheets, included in the 'STYLE' element, or put
into 'STYLE' attributes. The latter option offers easy
transition from HTML extensions.

new effects: some new visual effects have been added to offer
users new toys. The typographical pseudo-elements and the extra
values on the background property fall into this category.

scalability: CSS1 will be useful on equipment ranging from text
terminals to high-resolution color workstations. Authors can
write one style sheet and be reasonably sure that the intended
style will come across in the best possible manner.

CSS1 does not offer:

per pixel control: CSS1 values simplicity over level of control,
and although the combination of background images and styled
HTML is powerful, control to the pixel level is not possible.

author control: the author cannot enforce the use of a certain
sheet, only suggest

values, properties: we expect vendors to propose extensions to
the CSS1 set of values and properties. Extending in this
direction is trivial for the specification, but interoperability
between different UAs is a concern

layout language: support for two-dimensional layout in the
tradition of desktop publishing packages.

other DTDs: CSS1 has some HTML-specific parts (e.g. the special
status of the 'CLASS' and 'ID' attributes) but should easily be
extended to apply to other DTDs as well.

This document is a revised version of the CSS1 Recommendation which
was first published 17 December 1996 and the list below describes all
changes. By selecting the alternate style sheet "errata", all changes
will appear highlighted.

We would like to thank Komachi Yushi, Steve Byrne, Liam Quinn,
Kazuteru Okahashi, Susan Lesch, and Tantek Çelik for their help
in preparing this revised edition.

E.g., a style sheet can legally specify that the 'font-size' of an
'active' link should be larger that a 'visited' link, but the UA is
not required to dynamically reformat the document when the reader
selects the 'visited' link.

has been changed to:

E.g., a style sheet can legally specify that the 'font-size' of an
'active' link should be larger than a 'visited' link, but the UA is
not required to dynamically reformat the document when the reader
selects the 'visited' link.

typo4, typo5 [section 2.3-2.4] A trailing quote mark has been added to 'vertical-align'.